This invention relates to the continuous molten metal casting arts, and more particularly to wheel-band casting machines wherein the mold geometry is substantially square, rectangular, or trapezoidal. Certain embodiments of this invention may also be used with conventional twin belt or/and plate molds or other vertical casting machines which normally have a mold cavity containing sharp corners and thus form a cast bar having sharp edges.
It is well known in the prior art that combination of the following factors are closely interrelated to the presence or absence of bar cracks, especially corner cracks, in cast steel bar produced by typical metal casting machines: the composition of the metal or alloy being cast, especially alloys in which precipitation of some constituents is a problem; mold geometry and dimensions; speed of casting; overall cooling rate and uniformity of bar cooling. Of these factors, only the metal composition being cast is fixed by preference; except that the mold geometry cannot usually be changed during the casting operation but only when the operation is shut down, and then only with extreme difficulty. Many other parameters are variable within certain limits. With conventional wheel-band casting machines the trapezoidal shaped mold cross section with rounded bottom corners has long been recognized as the optimum shape for maximum wheel life, promotion of the desirable transverse solidification pattern, and ease of bar extraction from the mold (see for example U.S. Pat. No. 3,818,972 and 3,834,444 by Milton Berry and assigned to the assignee of the present invention). However, cast bars having trapezoidal or square cross sections undergoing even slight bending while hot may tend to produce longitudinal or transverse bar cracks with certain metal alloys such as steel, even when the cooling rates, or other combinations of parameters are varied over their permissable ranges.
These cracks are attributed to the difficulty of obtaining certain combinations of the above variable casting characteristics, especially heat transfer, casting rate, and alloy constituents, with a fixed mold geometry. It is known from the prior art that some factors are more important than others. For example, when certain constituent elements are precipitated to the grain boundaries, they then solidify at a cooler temperature than the predominant constituents. Due to the heat transfer characteristics of conventional square or trapezoidal molds, the intermetallic boundary in a cast bar forms along a longitudinal line generally extending from the corners toward the center, especially the sharp top corners of bars cast in the trapezoidal molds, thereby increasing the risk of bar cracking at this point. Various references in the prior art suggest changing the alloy composition to change the precipitation patterns or changing other factors such as cooling or casting rates to avoid such cracking. This invention is directed at alternate means to reduce or eliminate cracking, thus avoiding the complexities of chemical adjustment and eliminating the uncertainity of changing cooling characteristics for each of several alloys. It also permits continuous casting of otherwise difficult alloy combinations.
Further, in most prior art processes, mechanical corner preparation, such as scalping, is required to facilitate optimum rolling of the cast bar stock into rod, even if all parameters have been adjusted to minimize corner cracking, due to other common corner defects such as excessive porosity or excess flash or fins (i.e., material which has leaked between the wheel and band and solidified to form a thin metal fin) as illustrated in U.S. Pat. Nos. 3,780,552 and 3,469,620.
U.S. Pat. No. 3,520,352 discloses the use of insulating blocks or the like afixed external to the mold corners of plate molds used for vertical casting. However, in the use disclosed in that patent, the blocks are used solely to prevent expressive solidification stresses, and thus cracking, in the cast metal near joints of the plate mold which arise due to differential thermal contraction or expansion of the solidifying cast bar. Molten metal spills and breakthroughs can occur in such a casting method if no such precautions are taken to prevent corner cracking. The instant invention, however, is directed to an entirely different use for the introduction of a supplemental shaping material into the mold cavity itself.